This invention relates to an alloy for the occlusion of hydrogen, and more particularly to an alloy of four elements including a rare earth metal atom for the occlusion of hydrogen.
Hydrogen is an inexhaustible, clean substance capable of being easily transported and stored. As a new source of energy to take the place of fossil fuels, the hydrogen is attracting increasing attention.
Since hydrogen assumes a gaseous state at room temperature and liquefies at an extremely low temperature, development of hydrogen storage technology is urgently required. Recently, the technique of causing hydrogen to be occluded in the form of a metal hydride in a metal has been attracting keen attention as a promising approach to the effective storage of hydrogen.
The occlusion of hydrogen by the metal and the release of hydrogen from the metal are reversible reactions. These reactions proceed with liberation and absorption of a fairly large amount of heat of reaction. The pressures under which the metal occludes and releases hydrogen depend on temperature. Research is being carried out for the purpose of applying this principle to the space cooling and warming system or to the reversible thermal energy to pressure (mechanical) energy conversion system.
The requirements expected to be met by the metal used for the occlusion of hydrogen are that the metal should be available inexpensively and abundantly, that it should be easily activated and should possess a large capacity for the occlusion of hydrogen, that it should possess a suitable equilibrium pressure for hydrogen occlusion and release at or near room temperature, that it should involve little hysteresis of occlusion and release, that it should cause the reactions of hydrogen occlusion and release reversibly, that it should provide speedy hydrogen occlusion and release, and that it should be resistant to comminution.
Various materials have been proposed to the art as useful for the occlusion of hydrogen. Typical examples of such materials are LaNi.sub.5 and other RNi.sub.5 (wherein, R denotes a rare earth metal atom) and FeTi.
Although these alloys provide reversible reactions of hydrogen occlusion and release and possess large capacities for hydrogen occlusion, their reactions of hydrogen occlusion and release proceed slowly and their activation is not necessarily easy. Moreover, they involve heavy hysteresis and undergo comminution as the reactions of hydrogen occlusion and release are repeated. All these defects have stood in the way of actual use of these alloys.
The inventors formerly ascertained that alloys containing Misch metal provide effective occlusion of hydrogen and invented the following alloys: Mm.sub.1-x Ca.sub.x Ni.sub.5 alloy (U.S. Pat. No. 4,096,639), MmNi.sub.5-x Co.sub.x alloy (U.S. Pat. No. 4,147,536), MmNi.sub.5-x A.sub.x alloy (U.S. Pat. No. 4,222,770), MmNi.sub.5-x Cr.sub.x-y A.sub.y alloy (U.S. Ser. No. 192,809, dated Oct. 1, 1980), Mm.sub.1-x Ca.sub.x Ni.sub.5-y A.sub.y alloy (U.S. Ser. No. 222,351, dated Jan. 5, 1981), and MmNi.sub.5-x A.sub.x-y B.sub.y alloy (U.S. Ser. No. 276,833, dated June 24, 1981). These represent alloys of gradually improved properties for hydrogen occlusion. The Mm.sub.1-x Ca.sub.x Ni.sub.5 alloy has a low production cost but has a high equilibrium pressure for dissociation. The MmNi.sub.5-x Co.sub.x alloy has a lower equilibrium pressure for dissociation and a large capacity for hydrogen occlusion. The MmNi.sub.5-x A.sub.x alloy is improved in terms of speed of hydrogen occlusion and ease of activation. The MmNi.sub.5-x Cr.sub.x-y A.sub.y alloy offers constant pressure of dissociation over a wide range of hydrogen/metal atom ratios or a low flatness factor. The Mm.sub.1-x Ca.sub.x Ni.sub.5-y A.sub.y alloy is characterized by a particularly low cost. While the pressure for activation and the pressure for hydrogen occlusion generally fall in the neighborhood of 50 kg/cm.sup.2, the MmNi.sub.5-x A.sub.x-y B.sub.y alloy has succeeded in lowering these pressures.
These alloys, however, have large differences between the pressure for hydrogen occlusion and that for hydrogen release, namely large degrees of hysteresis. Thus, development of an alloy which is capable of providing effective hydrogen occlusion with little hysteresis has been longed for.
An object of this invention is to provide an alloy which possesses properties essential for the occlusion of hydrogen and involves only slight hysteresis.